Abstract

Since the 19th century, there has been disagreement over the fundamental question of whether
‘‘emotions’’ are cause or consequence of their associated behaviors. This question of causation
is most directly addressable in genetically tractable model organisms, including invertebrates
such as Drosophila. Yet there is ongoing debate about whether such species even have ‘‘emotions,’’
as emotions are typically defined with reference to human behavior and neuroanatomy.
Here, we argue that emotional behaviors are a class of behaviors that express internal emotion
states. These emotion states exhibit certain general functional and adaptive properties that apply
across any specific human emotions like fear or anger, as well as across phylogeny. These general
properties, which can be thought of as ‘‘emotion primitives,’’ can be modeled and studied in evolutionarily
distant model organisms, allowing functional dissection of their mechanistic bases and
tests of their causal relationships to behavior. More generally, our approach not only aims at better
integration of such studies in model organisms with studies of emotion in humans, but also
suggests a revision of how emotion should be operationalized within psychology and psychiatry.